Reading Tire Sizes and Sidewall Markings: The Basics

Understanding tire sidewall info doesn’t need to be complicated. And it’s important to know exactly what those numbers and letters mean.

When you’re shopping for a new set of tires, do you feel confused when the salesperson throws numbers and letters like 205/50/15 and M+S at you? It’s not just gibberish — these are important attributes telling you everything from whether the tire is the right size for your vehicle, to what environment it’s designed to drive in. And guess what? Everything’s written right on the rubber itself. Read on to learn how to decipher the most common markings.

Size

First off, all the information we will be discussing in this post is found marked on the outer side of each tire, appropriately called the sidewall. The size of the tire can be identified by the series of three numbers separated by a forward slash.

The first in the series denotes the width of the tire, measured in millimetres, from sidewall to sidewall.

The second number, called the aspect ratio, measures how tall the sidewall is, represented by a percentage of the width. For instance, a 50 means the sidewall is half as tall as it is wide. A shorter sidewall usually indicates the tire is more performance-oriented, the trade-off being a possibly bumpier ride.

The last number is the size of wheel that the tire will fit, in inches. The letter R sometimes precedes it, which is short for the industry design standard of radial construction.

graphic: Kal Tire

Load index and speed

A number on its own immediately preceding a letter is the load index and speed rating. The former refers to how much weight each tire can support, so a 94 means a maximum load of 1,477 pounds. The latter is the highest speed the tire can run at for an extended period, a V capable of sustaining up to approximately 240 km/h.

Treadwear

Somewhere on the tire you will likely find the word “treadwear” followed by a number: this is rating the lifespan of the tread before it’s replacement time. The benchmark is 100, so a 500 means the tire should have an estimated life of five times longer.

Traction

If you see a letter or letters marked either AA, A, B or C, these stand for how well or poor the tire’s traction is when braking in a straight line on a wet surface. AA is the best, and C is the worst.

Temperature

Either A, B, or C, the temperature rating grades the tire’s ability to withstand heat at high speeds.

Maximum air pressure

This is the maximum pounds-per-square-inch, or psi, of air pressure the tire can be inflated to. Different from the recommended tire pressure found in the vehicle’s user manual or on the driver’s side doorjamb.

Other

M+S, or mud and snow, indicates all-season capabilities and acceptable performance in most weather conditions. May also be represented by icons such as a sun, raincloud, snowflake, etc.

Benjamin Yong is a freelance writer and member of the Automobile Journalists Association of Canada. You can find his stories on this site and in other publications including RPM Canada, Westworld BC, CAA Magazine and Darpan Magazine.